The present invention relates to a process for producing an optical element comprising a polymeric liquid crystal layer having a helically twisted molecular structure. The invention further relates to an optical element obtained by the process, optical films using the optical element, and an illuminator and a liquid crystal display each using the optical element or optical films.
Conventional processes for producing an optical element comprising a polymeric liquid crystal layer having a helically twisted molecular structure include, for example, a process which comprises sandwiching a liquid crystal mixture comprising a polymerizable liquid crystal monomer and a chiral monomer between two parallel oriented substrates, and polymerizing and curing the mixture with ultraviolet rays or the like to produce a cholesteric polarizer having a helically twisted molecular structure (see Japanese Paten Laid-Open No. 6-281814). However, the process disclosed in this publication, in which a liquid crystal mixture sandwiched between two oriented substrates is polymerized, involves poor thickness precision and in-plane unevenness in optical properties due to the poor thickness precision. As a result, this process has disadvantages that brightness improvement is insufficient and front-view hue/slant-view hue characteristics decrease. In addition, there is a problem that a large amount of industrial wastes is unavoidable because of the necessity of oriented substrates on both sides of the liquid crystal mixture.
The cholesteric polarizers actually obtained and reported in the publication cited above have a thickness as large as from 18 to 20 xcexcm. Such a large product thickness results in poor thickness precision, which causes in-plane unevenness in optical properties. The related art technique hence has disadvantages that brightness improvement is insufficient and front view hue/slant view hue characteristics decrease. Still another problem is that the large product thickness increases the material cost.
Furthermore, the related art process described above requires exposure with an ultraviolet source (365 nm) at an irradiation intensity as extremely low as from about 0.06 to 5 mW/cm2 for a period of time as extremely long as from 5 to 60 minutes. This process hence has a considerably poor production efficiency. This related art process adds a dye to the liquid crystal mixture or disposes a spacer along the edges of the alignment substrates, and realizes the spread of a wavelength range of a cholesteric polarizer. In this case, however, orientation properties of the liquid crystal become insufficient and this causes in-plane unevenness in optical properties. This technique hence has the disadvantages that brightness improvement is insufficient and front view hue/slant view hue characteristics decrease. Moreover, in this related art process, the shearing operation with the two substrates for inhibiting the formation of discontinuity is conducted in such a manner that the two substrates are sheared over a short distance until planeness is obtained. This makes it difficult to conduct roll-to-roll production and the process hence has a considerably low production efficiency.
Accordingly one object of the present invention is to provide a process for producing an optical element which comprises a polymeric liquid crystal layer having a helically twisted molecular structure and has satisfactory thickness precision and satisfactory optical properties.
Another object of the present invention is to provide an optical element obtained by the process.
Still another object of the present invention is to provide optical films using the optical element.
Further object of the present invention is to provide an illuminator and a liquid crystal display each using the optical element or optical film.
As a result of intensive investigations to overcome the problems described above, it has been found that above-described objects can be accomplished with the production process described below. The invention has thus been completed based on this finding.
The present invention provides a process for producing an optical element comprising a polymeric liquid crystal layer having a helically twisted molecular structure, which comprises:
applying a mixed solution containing a polymerizable nematic liquid crystal compound and a polymerizable chiral reagent to an oriented substrate, followed by drying, to orient the mixed solution; and
irradiating the mixed solution applied with radiation from the oriented substrate side while maintaining the mixed solution in contact with a gas comprising oxygen to thereby polymerize and cure the mixed solution.
According to the process of the present invention, the mixed solution containing a liquid crystal material is oriented from one side thereof by one oriented substrate and then irradiated with radiation from the side opposite the mixed solution, i.e., from the oriented substrate side, while maintaining the mixed solution in contact with a gas comprising oxygen. This process improves orientation properties of the resulting liquid crystal layer in film thickness direction of the liquid crystal layer, and controls the rate of polymerization and curing by the difference in oxygen concentration. As a result, this process can attain an improvement in thickness precision and, hence, a decease in product thickness. This makes it possible to produce an optical element comprising a polymeric liquid crystal layer having a helically twisted molecular structure in the form of a thin film having a thickness of from 2 to 15 xcexcm. Further, the improved thickness precision brings about in-plane evenness in optical properties. As a result, brightness-improving characteristics and front view hue/slant view hue characteristics are improved, and this can realize the spread of a wavelength range of an optical element.
Furthermore, the process of the invention, in which one side orientation is conducted, is advantageous in that the amount of oriented substrates required is a half of that necessary in the related art process in which two oriented substrates are used to conduct both sides orientation, and thus attains decrease in the amount of industrial wastes. In addition, since high thickness precision and decreased product thickness can be attained, the cost of liquid crystal materials can be decreased. Still a further advantage of the invention is as follows. For the spread of a wavelength range of a dye-containing system in both-sides orientation embodiment, a certain extent of layer thickness is required for the reason that the rate of crosslinking reaction should be controlled based on light absorption by the dye. In contrast, according to the process of the present invention, orientation properties and thickness precision are improved, and hence, brightness-improving characteristics and front view hue/slant view hue characteristics are improved. Consequently, the optical element can be used in a wider wavelength range with the necessary and minimum thickness, whereby decrease in film cost can be attained. Moreover, roll-to-roll production is possible and improvement in production efficiency can hence be attained.
In the process for producing an optical element, the temperature during the irradiation with radiation is preferably 40xc2x0 C. or higher. By regulating the temperature during the irradiation with radiation to 40xc2x0 C. or higher, the liquid crystal material can be polymerized and cured in a satisfactorily oriented state. The temperature during the irradiation with radiation is more preferably from about 50 to 90xc2x0 C.
In the process for producing an optical element, the gas comprising oxygen preferably has an oxygen concentration of 0.5 vol % or higher. Air is preferably used as the gas.
The irradiation with radiation in the process for optical-element production is preferably conducted at an intensity of from 10 to 1,000 mW/cm2. According to the present invention, when an ultraviolet source (365 nm) is used at an irradiation intensity within the above range, an optical element can be produced through exposure in an extremely short period of time of from 0.1 second to 2 minutes. Thus, optical elements can be produced at an extremely high efficiency. The irradiation intensity is more preferably from 50 to 500 mW/cm2.
In the case where ultraviolet rays are used as the radiation in the process for producing an optical element, the mixed solution preferably contains a polymerization initiator in an amount of from 0.5 to 10% by weight based on the weight of the sum of the polymerizable nematic liquid crystal compound and the polymerizable chiral reagent. In order to polymerize the liquid crystal mixture by ultraviolet irradiation, the mixed solution should contain a polymerization initiator. The amount of the initiator used is preferably in the above range in order to obtain an optical element comprising a polymeric liquid crystal layer having a helically twisted molecular structure, that can be used in a wider wavelength range. The amount of the polymerization initiator used is preferably from 1 to 9% by weight, more preferably from 3 to 7% by weight.
In the process for producing an optical element, the polymerizable nematic liquid crystal compound is preferably a compound having one or more polymerizable functional groups, and the polymerizable chiral reagent is preferably a compound having polymerizable functional groups in a larger number than the polymerizable functional groups of the polymerizable nematic liquid crystal compound. When a polymerizable nematic liquid crystal compound and a polymerizable chiral reagent which satisfy that relationship with respect to the number of polymerizable functional groups are used, an optical element comprising a polymeric liquid crystal layer having a helically twisted molecular structure having higher suitability for a wider wavelength range can be obtained due to a difference in reactivity during the irradiation step. For example, when the polymerizable nematic liquid crystal compound is a compound having one polymerizable functional group, the polymerizable chiral reagent is preferably a compound having two or more polymerizable functional groups.
In the process for producing an optical element, the helically twisted molecular structure of the polymeric liquid crystal layer preferably has a difference in pitch between a part thereof on the side facing the oriented substrate and a part thereof on the opposite side. Furthermore, the pitch of the helically twisted molecular structure of the polymeric liquid crystal layer preferably changes so that the difference between the maximum pitch and the minimum pitch is at least 100 nm. By regulating the helically twisted molecular structure so that the pitch in a part thereof on the side facing the oriented substrate differs from the pitch in a part thereof on the opposite side, the polymeric liquid crystal layer can have suitability for a wider wavelength range. When the difference between the maximum and minimum pitches is regulated to that value, an optical element comprising a polymeric liquid crystal layer having a helically twisted molecular structure having especially high suitability for a wider wavelength range is obtained. The polymeric liquid crystal layer is preferably one which has uniform optical properties and in which the pitch of the helix gradually changes continuously from the minimum value for one side of the layer to the maximum value for the other side of the layer. This change in pitch exerts a favorable influence on the strength of the layer.
The present invention further provides an optical element obtained by the process for optical element production described above. The optical element obtained is used as, for example, a selective reflection type polarizing optical element. Other uses of the optical element include a retardation film (optical compensating film), twisted retardation film, and oblique retardation film.
The present invention furthermore provides an optical film (cholesteric polarizer) comprising a combination of the optical element and a retardation film, and an optical film comprising a combination of the optical element and an absorbing polarizer film.
The selective reflection type polarizing optical element or the like or an optical film using the same can be used in an illuminator which comprises a surface light source having a reflecting layer on the back side thereof. In this illuminator, the optical element or optical film is disposed on the front side of the surface light source. The illuminator preferably has at least one prism array layer. This illuminator preferably has two or more prism array layers disposed so that the direction of array arrangement in one of the layers intersects that in the adjacent layer(s).
The present invention still further provides a liquid crystal display which comprises the illuminator and a liquid crystal cell disposed on the light emission side of the illuminator. In each of the optical films, illuminator, and liquid crystal display described above, all or part of the constituent layers may have been tightly bonded through an adhesive layer.